This invention relates to an adaptive noise canceler for use in, for example, a hands-free automobile telephone.
Much research has recently been done on the removal of background noise such as engine noise from a voice signal picked up by the microphone of an automobile telephone. One method that has been proposed employs two microphones, one disposed close to the voice source and far from the noise source, the other disposed far from the voice source and close to the noise source. During an interval in which background noise is present but voice is absent, a switch is depressed, causing the input levels from the two microphones to be compared and their ratio to be set as a threshold value. When voice is present, an adaptive filter uses this threshold to remove the noise component.
A major problem in this method is that whenever the background noise level changes, the operator must depress the switch to establish a new threshold level. Changes in noise level occur frequently in the cabin of an automobile: the vehicle speed varies, the engine rpm rate varies, traffic conditions vary, windows are opened and closed, the air conditioner is switched on and off, and so on. Without some means of automatically adjusting to different noise levels, the method described above has little practical value.
Another problem is that it is difficult to set an appropriate threshold when the signal-to-noise ratio is low.